Power amplifiers (PAs) amplify an input signal and provide an amplified output signal having a significant amount of power. Radio Frequency (RF) PAs may provide the amplified output signal to an antenna or to another power amplifier having a higher output power.
An RF PA may be required to operate with a high linearity, such as when regulations limit the transmitted power and distortion produced by the RF PA. The RF PA may also be required to operate with high efficiency, in order to minimize the amount of waste heat generated by the RF PA, prolong the duration of an operation of a battery powered device, or both.
When the output of the RF PA is provided to an antenna, the impedance of the antenna may vary substantially because of environmental conditions. For example, the impedance of a cellular telephone antenna may vary substantially depending on whether and how the cellular telephone is being held. The variations in the impedance of the antenna may cause a higher voltage to be present across components of the RF PA, which may damage the RF PA.
The RF PA may be used to transmit in a number of different frequency bands, and in a number of different modes. The power and linearity requirements of each frequency band and each mode may be different from the power and linearity requirements of other frequency bands and other modes.